1. Field of the Invention
The present invention relates to an image processing method that enables reducing damage to a thermally reversible recording medium attributable to repeated recording and erasing of each image and preventing deterioration of the thermally reversible recording medium and also relates to an image processor that can be suitably used for the image processing method.
2. Description of the Related Art
Each image has been so far recorded and erased on a thermally reversible recording medium (hereinafter, may be referred to as “recording medium” or “medium” merely) by a contact method in which the thermally reversible recording medium is heated by making contact with a heat source. For the heat source, in the case of image recording, a thermal head is generally used, and in the case of image erasing, a heat roller, a ceramic heater or the like is generally used.
Such a contact type recording method has advantages in that when a thermally reversible recording medium is composed of a flexible material such as film and paper, an image can be uniformly recorded and erased by evenly pressing a heat source against the thermally reversible recording medium with use of a platen, and an image recording device and an image erasing device can be produced at cheap cost by using components of a conventional thermosensitive printer.
However, when a thermally reversible recording medium incorporates an RF-ID tag as described in Japanese Patent Application Laid-Open (JP-A) Nos. 2004-265247 and 2004-265249, the thickness of the thermally reversible recording medium is naturally thickened and the flexibility thereof is degraded. Therefore, to evenly press a heat source against the thermally reversible recording medium, it needs a high-pressure. Further, when there are convexoconcave or irregularities on the surface of a thermally reversible recording medium, it becomes difficult to record and erase an image using a thermal head or the like. In view of the fact that RF-ID tag enables reading and rewriting of memory information from some distance away from a thermally reversible recording medium in a non-contact manner, a demand arises for thermally reversible recording media as well. The demand is that an image or images be rewritten on such a thermally reversible recording medium from some distance away from the thermally reversible recording medium.
To respond to the demand, a recording method using a non-contact laser is proposed as a method of recording and erasing each image on a thermally reversible recording medium from some distance away from the thermally reversible recording medium when there are convexoconcave or irregularities on the surface thereof.
As such a recording method using a laser, a recording device (laser maker) is proposed of which a thermally reversible recording medium is irradiated with a highly energized laser beam to control the irradiation position. A thermally reversible recording medium is irradiated with a laser beam using the laser marker, the recording medium absorbs light, the light is converted into heat, a phase change is generated on the recording medium by effect of heat, thereby an image can be recorded and erased.
The laser marker is configured to record each image by irradiating a region to be recorded with a laser beam by scanning the laser beam while changing a laser beam irradiation direction by changing a scanning mirror angle with motor actuation. Thus, the scanning speed is decelerated due to acceleration and deceleration operations during a time period from a stopped state of the scanning mirror until the scanning mirror begins to be actuated or during a time period from an actuated state of the scanning mirror until the scanning mirror is stopped. For this reason, at a recording start point (a start point), a recording end point (an end point), and a folding point where the rotational direction of the scanning mirror is changed, the scanning speed of the scanning mirror is lowered, and an excessive amount of energy is applied to these portions. Therefore, there is a problem that a thermally reversible recording medium is damaged by repeatedly recording and erasing an image. Further, when scanning a laser beam using an XY stage instead of a scanning mirror, the scanning speed is decelerated due to acceleration and deceleration operations during a time period from a stopped state of the XY stage until the XY stage begins to be actuated or during a time period from an actuated state of the XY stage until the XY stage is stopped. For this reason, similarly to the case of using a scanning mirror, an excessive amount of energy is applied to a start point and an end point of a recorded image, and there may be cases where the thermally reversible recording medium is damaged.
On these points, even when an excessive amount of energy is applied to a conventional non-reversible heat-sensitive recording medium, this does not become a major problem, however, on a thermally reversible recording medium where each image is repeatedly recorded and erased, there is a large problem that an excessive amount of energy is applied to the same portions to cause damage to the recording medium, and each image cannot be uniformly recorded at high-image density and cannot be uniformly erased due to accumulation of damage.
To solve these problems, for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-127446 describes that when an image is recorded on a thermally reversible recording medium so that record dots overlap each other or when an image is recorded with folding lines, laser irradiation energy is controlled for every imaging points to reduce energy to be given to these portions; and also describes that when straight lines are recorded, local thermal damage is reduced by reducing energy at every certain intervals to thereby prevent deterioration of the thermally reversible recording medium.
Japanese Patent Application Laid-Open (JP-A) No. 2004-345273 describes a technique of reducing energy by multiplying irradiation energy by the following expression in accordance with an angle R where a laser beam angle is changed when an image is recorded using a laser.|cos 0.5R|k(0.3<k<4)
With use of this technique, it is possible to prevent an excessive amount of energy from being given to overlap portions in line images when an image is recorded using a laser and to prevent deterioration of a recording medium or to maintain an image contrast without excessively reducing the energy.
Further, Japanese Patent Application Laid-Open (JP-A) No. 2006-306063 proposes a recording method in which when a certain image is recorded by irradiating a non-contact type rewrite thermal label with a focused laser beam, a light scanning device is continuously driven without oscillating the laser beam, and only when a trajectory of the laser beam assumed when the laser beam is oscillated (a virtual laser beam) moves at a substantially constant speed, the laser beam is oscillated to scan the laser beam and to record the image on the non-contact type rewrite thermal label.
These conventional recording methods respectively provide a technique in which an excessive amount of thermal energy is not to be applied to a thermally reversible recording medium at overlap portions when recording an image using a laser. However, when a uniform image is recorded at high-density and erased repeatedly by using a highly energized laser, not only a start point, an end point and a folding portion of an image line but also the center portion of a straight line are excessively heated, deformed sites and air bubbles are observed on the surface of the thermally reversible recording medium, and materials themselves each taking a roll of color developing-color erasing properties are thermally decomposed, and these materials cannot exert their sufficient ability. As a result, on the entire image lines including start points, end points, folding portions and straight lines constituting an image, it is impossible to uniformly record the image with high-image density and is impossible to uniformly erase the image on a sufficient level, and as an image processing method that causes less deterioration of a thermally reversible recording medium even when the image is repeatedly recorded and erased, there is much to be desired, and further improvements and developments are still desired.